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Using Variable Variables in PHP

Quite possibly one of PHP’s least utilized yet most valuable features is its support for
Variable Variables. In short these are a method of using the value of one variable to call
the name of another. Sound confusing? We’ll explain everything…Quite possibly one of PHP’s least utilized yet most valuable features is its
support for Variable Variables. I stumbled across this concept just recently
while working on a “Fiction Portal” project for a gaming website. Users of the
service could submit their own fictional stories to a program which would then
dump them into a MySQL table and populate that table’s columns with the
characteristics of the story that the author provided. The way the table had
been setup was such that each possible characteristic of a story was a separate
column, and each story that held that characteristic would hold a value of 1 in
the column whereas each story that didn’t would hold a value of 0 in it.
Therefore, a sample story might look like this in the table:

Everything went smoothly until I discovered that this
method of sorting made searching for stories with only certain criteria nearly
impossible. Cue Variable Variables.

{mospagebreak title=Variable Variables?} A variable variable is, in short, a method of using the value of one
variable to call the name of another. For instance, if I had a variable $x with
the value of “Eric”, I could set a variable $Eric to “Seufert” by simply writing
$$x = “Seufert”. This process calls the value of $x and then creates a variable
out of it. Therefore,

echo “$x ${$x}”;

would produce the same output as,

echo “$x $Eric”;

which is:

Eric Seufert

Notice the curly braces around the variable variable in
the first echo statement. This is the standard syntax for outputting variable
variables. Omitting the curly braces would have resulted with the following
output:

Eric $Eric

{mospagebreak title=Using Variable Variables with Arrays} Variable Variables make working with forms and other such user-input
easier because they allow you to centralize all the available options. Getting
back to my aforementioned problem, I had to figure out a way to select only the
stories with the criteria that the person selected. The easiest way to go about
this was to create an array to hold only the stories that fit the criteria the
user specified and then create a MySQL query string with the values. Here’s the
code I used:

(*note* The system running the program did not have PHP
4.0 installed at the time, which is why I did not just use array_push to add
values to the array in the first loop)

The actual values of the variable
variables created from the $atts array having been already passed to the
program, everything was accomplished in a few lines of code rather than
replicating the same process for each different characteristic, showing the
efficiency of variable variables when used with forms.

Building arrays
from variable variables is equally simple. If, perhaps, I wanted to include my
middle and last name within the $$x variable, I could initialize it as an array
with the following code:

$$x = array(“Benjamin”, “Seufert”);

However, array elements cannot be directly accessed in
variable variable form, so outputting my entire name could be handled in one of
two ways:

$temp1 = ${$x}[0];
$temp2 = ${$x}[1];
echo “$x $temp1 $temp2″;

or,

echo “$x $Eric[0] $Eric[1]“;

both of which would print,

Eric Benjamin Seufert

Had I tried to directly print the variable variable array
elements, I would have received the following output:

Eric Array[0] Array[1]

{mospagebreak title=Variable Variables with Functions} A great yet widely unused element of variable variables is their
ability to be employed with functions. The function name can be set within a
variable and then run through calling the variable variable of that variable.
Here’s an example:

$function1 = “sort_stuff”;
${$function1()};
#this sets the result of the sort_stuff() function into a variable
${$function1($var1, $var2, $var3)};
#this does the same thing but sends the function parameters

Here’s a quick example of this in action, building on the
same program segment I had introduced variable variables with:

as the echo statement is printing the value of the
variable that gets created from the resultant of the value() function, “Eric”.
As we have already set $$x (which, following the variable variable, is the same
as $Eric) to “Seufert”, the same value is outputted, only this time resulting
from a function call.

{mospagebreak title=Variable Variables with Classes} Variable Variables work equally well with class objects, the basic
syntax of which is almost the same as the use of variable variables with
functions. Set the class name into a variable, call the variable variable of
that variable and you then have access to variables within that class. Here’s an
example:

{mospagebreak title=The Never-ending Variable}
Another interesting quirk about variable variables is that they are seemingly
infinite. The following code works just fine to produce “The Great Thing About
Variable Variables Is They Never End!”:

{mospagebreak title=Conclusion} Variable Variables
are one of those little workarounds that make writing code that much easier.
Hopefully you have picked up some helpful new techniques from this article to
carry with you into practical, everyday use.